Some magnetic position tracking systems track the position of a catheter or other probe in a patient's body by generating known magnetic fields and measuring the fields using a magnetic field sensor fitted in the probe. Systems of this sort are described, for example, in U.S. Pat. Nos. 5,391,199, 6,690,963, 6,484,118, 6,239,724, 6,618,612 and 6,332,089, PCT International Publication WO 96/05768, and U.S. Patent Application Publications 2002/0065455, 2003/0120150 and 2004/0068178, whose disclosures are all incorporated herein by reference.
U.S. Pat. No. 6,177,792, whose disclosure is incorporated herein by reference, describes an apparatus for generating magnetic fields, including a plurality of radiator coils and driver circuitry coupled thereto. The driver circuitry drives the coils so as to generate magnetic fields at a plurality of driving frequencies, wherein each of the plurality of radiator coils generates a field substantially only at a single, respective driving frequency. Circuitry is associated with at least one of the plurality of radiator coils for substantially eliminating magnetic fields generated by the other coils.